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Aviation History
1938
1938 - 2418.PDF
SUPPLEMENT TO FLIGHT I74rf 56 THE AIRCRAFT ENGINEER AUGUST 25, 1938 i/A :)0 • 40 50 (W 70 80 90 108 110 120 130 140 ISO 10 1.40 1.30 1.21 1.09 1.00 .91 .81 .72 .62 .51 42 .32 .21 15 1.3B 1.27 1.19 1.00 — 83 .74 .66 56 .47 .38 29 TABLE 2. HOOP SPACING FACTORS. 20 1.32 1.21 1.17 1.08 1.00 .92 .85 .77 .70 .62 .53 .45 .37 % Reinforcement. 25 1.28 1.21 1.15 1.07 1.00 .93 .80 .79 .73 66 .58 .51 .44 30 1.24 1.18 1.13 1.06 1.00 .04 .88 .82 .76 .70 .04 .58 .52 35 1.22 1.10 1.11 1.05 1.00 — .89 .83 .78 .73 .08 .63 .57 40 1.20 1.14 1.09 1.01 1.00 .95 .90 .85 .81 .76 .72 .68 .63 45 1.18 1.13 1.08 — 1.00 — — .86 .82 .78 .74 .71 .67 50 1.17 1.12 1,07 1.03 1.00 .90 .91 .87 .84 .80 .77 .74 .71 NOTE.—Th> radius of gyration of the stiffener is calculated about the axis XX Fig I increasing reinforcement, other features being constant. Hence 50 per cent, reinforcement has been adopted as the upper limiting value. In Table 2, again obtained by interpolation, I have tabulated the effects ot hoop spacing ELS governing the slenderness ratio of the longitudinal stringers. It will be noted that the results are given in form of factors with slenderness ratio of 70 as unity. In Table 3 the effects of curvature of the skin have been assessed in a somewhat similar manner, the flat plate being considered as unit factor. The routine application of the data is as follows :— (i) From Table 1, for skin thickness and percentage 150 200 250 300 350 400 . 450 500 550 600 700 800 900 Flat 10 2.40 1.73 1.45 1.32 1.27 1.22 1.21 1.19" 1.15 1.13 1.09 1.05 1.02 1.00 15 2.23 1.66 1.41 1.30 1.25 1.21 1.20 1.18 — — — — — 1.00 TABLE 3. SKIN CURVATURE FACTORS. 20 2.06 1.59 1.38 1.28 1.24 1.20 1.19 1.17 1.14 1.12 — — — 1.00 % 25 1.89 1.52 1.34 1.26 1.22 1.19 1.18. 1.16 -— — 1.08 —•• — 1.00 Reinforcement. 30 1.72 1.45 1.31 1.24 1.21 1.18 1.17 1.15 1.13 1.11 — — — 1.00 35 1.56 1.38 1.27 1.22 1.19 1.17 1.16 1.14 •— .— — — — 1.00 40 1.40 1.31 1.24 1.20 1.18 1.16 1.15 1.13 1.12 1.10 1.07 1.04 1.01 1.00 45 1.24 1.22 1.20 1.18 1.16 1.15 1.14 1.12 — — — 1.00 50 1.08 1.13 1.16 1.16 1.14 1.14 1.13 1.11 1.11 1.09 LOO 1.00 TABLE 4. .SUMMARY OF item. Skin thickness, 24 S.W.G. Effective length of skin. Four 5.6in. pitches Area of effective skin ... Stringer thickness, 24 S.W.G Developed width of stringer Area of one stringer No. of effective stringers Total area of effective stringers Percentage reinforcement Basic stress from Table 1 Radius of gyration of stringer about xx Slenderness ratio of stringer ... Hoop spacing factor from Table 2 Skin curvature ratio ... Curvature factor from Table 3... Predicted unit stress Load ... ... ... • Developed stress _. RESULTS. Test. No. 1. .022in. 22.4in. .493 sq. in. .022in. 2in. .044 sq. in. 4 .176 sq. in. 26.3 7,260 Ib./sq. in. .167in. . 112 .73 705 1.08 5,720 lb./sq. in. . 1.35 tons 7,690 Ib./sq. in. No. 2. .022in. 22.4in. .493 sq. in. .022in. 2in. .044 sq. in. 8 .352 sq. in. 41.7 11,680 lb./sq. in ,167in. 112 .81 705 1.07 10,130 lb./sq. in. 1.98 tons 8,950 Ib./sq. in. NOTE.—R ^ : Internal radius of skin, in inches. Thickness of skin, in inches. reinforcement obtain compressive stress for flat sheet. (ii) From Table 2, for. hoop spacing modify stiffener slenderness ratio to obtain appropriate factor, (iii) From Table 3, for skin curvature obtain appropriate factor. (iv) To predict probable- final unit compressive stress multiply (i) by (ii) by (iii). We will now compare predicted with test results. The arrangement of the test specimen is shown in Fig. 2, and the results are summarised in Table 4. In the case of a circular or elliptical section fuselage there is no obvious point at which the shear panel can be considered to begin and the Curtiss data makes no reference to effective boom material. An approximation is illus trated in Fig. 2, the procedure being to inscribe a circle with the minor axis of the ellipse as diameter. From the intersection of the axes draw two radials, one on each side of and at 30° to the major axis. Project parallel to the major axis the intersections of radials and inscribed circle. The intercept on the ellipse circumference of these two projections is considered as the effective length of reinforced skin under boom load. The results from Test No. 1, Table 4, would appear to indicate that the Curtiss estimate is on the pessimistic side, but it should be noted that the stringers used in the test were of considerably more efficient cross-section. The apparent discrepancy in Test No. 2 between predicted and developed stress is due to the fact that test was discontinued when the specified proof factor was realised. There was then no sign whatever of incipient failure. So on the whole the Curtiss method appears to furnish quite a fair indication of the probable developed stress. TWO AMERICAN TEXTBOOKS ON STRUCTURES " Airplane Structures,"' Niles and Newall, Vol. I, Chapman and Hall, 25s. " Metal Airplane Structures," Loudy,' Constable, 24s. IT is good to have the up-to-the-minute " Niles and Newall " in a second edition. The original edition, nine years ago, was excellent, but its usefulness decreased a little as time passed and aircraft design moved on. But the book has now caught up again and includes the methods of stress analysis for monocoque and stressed-skin construction. It is really a complete textbook on the theory of struc tures in which the examples are principally chosen from aero plane design rather than from cranes and the Crystal Palace. No basic principles are skipped, leferences are quoted and authorities given. The use of mathematical proofs and the methods of the calculus are not avoided in such a way as to insult the intelligence of the reader. After putting the problem in true perspective by giving an introductory chapter on the general procedure of aircraft design, the authors take one through the loading conditions and American stressing cases. They then give very complete chapters to Bending Moments, Shears, and Influence Lines; Deflections ; Continuous and Restrained Beams ; lorsion ; Truss Analysis by the method of moments, the use of tension co efficients, and by graphical means; the design ot simple Ties and Columns; Joints and Connections; and, finally, Strain- Energy. *" Each chapter has many worked examples using typical aeroplane structures of tube, timber, and sheet metal where appropriate. At the end of each chapter are many problems on which the student may practise. We can recommend this book thoroughly, both to young students and the older generation who need a refresher. We are not so whole-hearted about Major Loudy's book on "Metal Airplane Structures." A price-for-price comparison with Niles and Newall leaves no doubt as to which is better value. It appears to consist principally of expurgated sum maries from N.A.C.A. reports and technical notes, etc., with very little continuity and explanation. The illustrations are inferior and the examples of typical metal aeroplanes are those of ten years ago. The Supermarine "Southampton" is described as an ex cellent example of British construction (which it was in 1926) and the only Sikorsky boat mentioned is the S40. For those without access to the American N.A.C.A. and other official publications this book may have some value. But they should know something of the subject already
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